Frequency conversion



Nov, 28, 1944.

H. M. CROSBY FREQUENCY CONVERSION Filed A ri; 1, 1943 2 Sheets- Sheet 1 NA AA vvv pvvv

VVVVL In YTC'OYI Howa His Attorney.

M. Crosby,

H. M. CROSBY 2,363,835

FREQUENCY CONVERSION Nov. 28, 1944.

Filed April 1, 1943 2 Sheets-Sheet 2 rm: ANQDE up; use

Inverfitor: Howard. M. Crosby,-

His Attorney.

estems Now. as

' unit snares ildldtfihii IFREQETENUH @ilii iifhfifiiiiii Howard id. Crosby, Schenectady, E ses-le ner "to General Electric iilosnpany,

New York a corporation oi Application April 1, Wilt, Serial No. Mild l i Claims. 7 tea. some) My invention relates to frequency conversion, and more particularly to balanced or push-pull frequency converters having signal means responsive to a deviation of the heterodyne frequency from a selected normal value.

Frequency mixers or converters are an integral oi the hetercdyne frequency of a frequency converter from a desired normal value.

My invention itself will be more fully understood and its objects and advantages better appart of superheterodyne receivers in which the incoming signal modulated radio frequency wave is mixed with a, locally generated oscillation of different frequency to obtain a modulated wave of intermediate frequency. Such mirrors are also used in transmitting apparatus to supply a carrier wave of desired frequency for modulation. In transmission of signals the use of a heterodyne converter for supplying a wave of carrier frequency has the advantage of flexibility over i a simple crystal oscillator in that the frequency of the carrier wave may be varied over a range illustration to show my invention applied to the of selected values. Furthermore, if a crystal oscillator is used to supply one of theinput oscillations to a heterodyne converter in a radio transmitter, the arrangement is superior to a simple vacuum tube oscillator in that an uncontrolled variation in the frequency of the vacuum tube input oscillator will produce, on a percentage basis, a smaller change in the dlfl'erence or output frequency. Thus the combination in a mixer of a wave of fixed crystal frequency and a wave of variable vacuum tube oscillator fre quency provides means for combining the flexibility of the vacuum tube oscillator, at least in part, with the constant frequency characteristic of the crystal oscillator.

In certain applications, however, absolute constancy of output frequency of such a converter is necessary. Such constant frequency output is (always desirable .in both a superheterodyne receiver and in the carrier wave generator of a trarsmitten-or wherever a frequency converter is use Accordingly, it is a general object of my invention to provide means for detecting a deviation-from a desirednormal value of the superheterodyne frequency in a frequency converter.

It is a further object of my invention to provide a new and improved balanced or push-pull frequency converter having means for detecting deviation of the heterodyne irequencyfrom a desired normal value.

Itis a specific object of my invention to provide a normally null signal means responsive to difference frequencies within both the audible and visual ranges and down to one cycle per second or less to indicate the extent oi departure carrier wave generator of a radio transmitting apparatus. in the'drawings 9, source I0 of electric oscillations of fixed frequency and a second source I I of electric oscillations of a diflerent frequency and subject to uncontrolled frequency variations Jointly supply energy to the current control grids of a pair of push-pullconverter or mixer tubes 12 and l3.- The-tubes l2 and I3 supply oscillating electricenersy at the sum and difierence frequencies to an output transformer Id. The source of oscillations iii of fixed frequency may suitably comprise a piezoelectric crystal oscillator connected to supply oscillations at fixed frequency and in like phase to a pair of control grids iii and ii in electric discharge tubes i2 and iii, respectively. The source Ilmay be a vacuum tube oscillator of any welllznown type the frequency oi which is controllable, and any selected frequency of which is subject to slight uncontroiled variations. The

. source [I is connected to supply electric oscillations in phase opposition to a pair of suppressor grids l9 and it in the electric discharge tubes 12 and i3, respectively. In the example chosen by way of illustration, namely, a frequency converter for supplying an unmodulated carrier wave to a transmitting apparatus, the oscillator ii will supply an unmodula'ted wave to the grids l9 and 20 and the intermediate frequency output wave appearing at the transformer M will be supplied to a modulator where it may be modulated, either in amplitude or in frequency, at a signal frequency representative of the intelligence being transmitted. Preferably the free quency of the oscillator ill is relatively high compared to the frequency of the oscillator II. By way of example it will be assumed that the fixed frequency of the source ii) is ten times. the desired normal frequency of source ll.

The electric discharge devices it and I3 are also provided with anodes 30 and SI and cathcathode of each discharge tube are connected in separate output circuits both of which are supplied from a common source of direct electric current supply such as a battery 34. The separate output circuits of the discharge tubes l2 and I3 are connected to the battery 34 in opposing or push-pull relation, and each includes onehalf of the primary winding of the output transformer [4, one of the discharge tubes l2, l3, l0

and one of a pair of similar resistors 35, 33.

The positive. terminal of the battery 34 is connected to the midtap of the primary winding 'of the transformer l4, while the negative terminal of the battery 34 is connected to the resistors tive potential with respect to the potential of The screen grids 31 the cathodes 32 and 33. and 33 serve electrostatically to isolate the control grids l3 and I! from the suppresser grids l3 and 20, respectively.

The resistors 35 and 33 each have-one end connected to the negative terminal of the battery 34 and to ground at 39. For the purpose of preventing the passage of relatively high frequency currents through the resistors 35 and 33,'the resistors are shunted by high frequency by-pass condensers 40 and 4|; The opposite ends of the resistors and 33 are connected through high frequency choke coils 42 and 43 to a suitable signal frequency amplifier 44. The output of the signal amplifier 44 is supplied in parallel circuit 35 relation to both a suitable audible signal reproducing device, such as a headphone 45, and to a visual signal receiving apparatus, such as an electric discharge tube 45. The discharge tube odes 32 and 33, respectively. The anode and appears across the secondary winding of the transformer l4 because of the relatively low impedance which the tuned transformer circuit offers to oscillations at the sum frequency. It will I also be noted that currents at the frequencyoi the source l0 do not appear in the secondary winding of the transformer l4 because of the fact that such currents flow in like phase relation and in opposite directions on opposite sides of the midtap of the primary winding of the transformer I4. Potentials at the frequency of the source ll, since they are supplied to the converter tubes 12 and I3 in opposing phase relation, induce potentials of like frequency in the secondary winding of the transformer l4. However, since the frequency of the source I l is wide- 'iy different from the difference frequency for which the output circuit of the transformer I4 is tuned, the impedance of the output circuit for currents of the frequency of the source H is very low and substantially no voltage of this frequency appears across the transformer secondary winding.

Let it now be assumed that there is present in the output of the vacuum tube oscillator ll an appreciable amount of tenth harmonic oscillating energy. If then the frequency of the tube oscil-' lator II is precisely one-tenth the fixed frequency of the crystal oscillator l0, which is the desired condition assumed, the tenth harmonic of the tube oscillator II will be of precisely the same frequency as the crystal oscillator output. The tenth harmonic of the source H, as well as the fundamental oscillation from this source, will be mixed by the tubes 12 and I3 with the crystal output from the source I!) to provide sum and difference frequencies in the output circuits of each of the tubes. When the frequency of the source II is properly adjusted, the difference fre 46 is preferably a visual tuning indicator tube of fluency between its tenth harmonic a the the type described and claimed in Patent 2,051,- 189 issued to H. M. Wagner on August 18, 1936. Briefly, the tube 43 comprises an evacuated envelope containing a cathode 41, an anode 48, a

control grid 43, and a target electrode 50. For

indicating purposes a suitable source of direct current electric energy, such as a battery 5|, has its positive terminal connected directly to the target electrode 50 and through a resistor 52 to the anode 48. The negative terminal of the bat- 50 tery 5| is connected to the cathode 41'. The signal oscillation from the output of the amplifier '44 is connected between the cathode 41 and the control grid 49 of the tuning indicator tube 43.

The target electrode 53 is in the form of a conical 55 luminescent screen upon which is formed a circular image the circumferential length of which is directlyproportional to the voltage between the cathode 47 and the control grid 43.

From the foregoing detailed description of the organization and arrangement of the apparatus,

the'operation of my invention will be understood from the following description; In the first place it will be readily apparent that the balanced or push-pull mixer circuit itself operates in a well The transformer I4 is crystal frequency will be zero, so that a prede-- termined constant average direct current will be present in the output circuits of each of the mixer tubes [2 and I3. The sum frequency of the crystal and the tenth harmonic of the vacuum tube oscillator is very high and is by-passed through the condensers 40 and 4| which offer very low impedance to high frequencies. These condensers serve also to by-pass any components of the crystal frequency itself, the fundamental frequency of the vacuum tube oscillator, and the sum and difference frequencies of the crystal and the vacuum tube oscillator fundamental which may appear in the cathode resistor circuits of the mixer tubes. I2 and I3. Thus so far as the resistors 35 and 33 are concerned, only the difference frequency of the crystal'oscillafror and the tenth harmonic of the vacuum tube oscillator need be considered. Currents of this difference frequency fiow through the two output circuits of the tubes i2 and I3 from the positive terminal of the battery 34 through separate halves of the primary winding of the transformer M in opposite directions to the anodes of the tubes l2 and i3, and through the tubes l2 and I3 and the resistors 35' and ,35 to ground and the negative terminal of the battery 34. Thus if the resistors 35 and '33 are of equal resistance, the capacitors 40 and 4! of equal capacitance, and the tubes I 2 and I3 of similar electrical characteristics, the same average direct current potential exists at the electrically spaced positive terminals 55 and 53 of the resistors 35 and 33, respectively. Under these conditions the input terminals of the signal Substantially no voltage at the sum frequency amplifier 44 are supplied with the same potential,

and the output of the amplifier is zero, so that no signal indication is discernible either through i Let it now be assumed that the frequency of the vacuum tube oscillator H changes slightly so that the tenth harmonic of the new frequency is no longer the same as the fixed frequency of the crystal source l0. Referring now to Fig. 2, I have shown a group of diagrams illustrating the grid voltages of the mixer tubes I2 and I3 under this condition. In this diagram the curve Em represents the voltage from the source l applied to the control grid 16 of the electric discharge tube 12, and the curve E11 represents the same voltage applied to the control grid II of the discharge tube I3. It will be noted that these voltages are identical in phase and magnitude. For the purpose of illustration a frequency of four cycles per second has been assumed for E16 and Em and for the normal value of the tenth harmonic from the source II. If it is assumed now that the tenth harmonic of the vacuum tube oscillator II has changed to three cycles per second, it will be evident that the curve E19 represents the voltage of the suppresser grid 19 in the tube l2, while the voltage E20 represents the voltage simultaneously applied to the suppresser grid 20 in the tube II. It will be noted that these voltages are in phase opposition. Since the suppresser grid and control grid characteristics of the tubes l2 and I3 are qualitatively similar, the current controlling effect of separate voltages on these grids is similai'fto the effect of the sum of the voltages upon a single grid. Thus the equivalent grid voltage supplied to the mixer tube'l2-is the sum of E19 and Em. This voltage has been illustrated by the curve E12. Similarly. the curve E13 represents the equivalent grid voltage applied to the mixer tube l3. It will be noted that these net grid voltages vary in magnitude in like manner but in opposite phase as a result of beating together the identical voltages Em and E17 with the oppositely phased voltages E19 and E20.

Referring now to Fig. 3, I have shown a repre- I sentative grid characteristic curve E; which is qualitatively similar to the characteristic of both the control grids and the suppresser grids of the mixer tubes i2 and i3 and represents the characteristic of an assumed equivalent single grid in each tube. It will be understood that the assumption of an equivalent grid has been made merely in the interest of simple qualitative explanation,

and that the operation is subject to more precise analysis. It is well known that the lower portion of the curve E8 is nonlinear and that if the average grid bias is maintained in this region of the curve, the tubes i2 and I3. function as detectors. I have shown a line Egb which represents the average negative grid bias resulting from the connection of the resistors 35 and 36 between the cathodes and control grids of the tubes l2 and It. Upon the line Egb I have drawn the grid voltage curves E12 and E13 taken from Fig. 2. It will now be clear to those skilled in the art that due to the grid characteristic of the tubes [2 and I3 as illustrated by the curve Eg, the anode current of the tube i2 is represented by the curve I12, while the anode current of the tube I3 is represented by the curve In. Now it may be readily observed that due to the nonlinearity of the curve Eg the positive half-cycles of the curves I12 and IE3 are considerably more amplified than are the negative half-cycles, so that the'average of the curve I12 is represented by a curve I35, and the average of the headphone 45 or the visual tuning indicator ,48.

r harmonic of the source ll.

from Figs. 2 and 3 that where the difference freand I1: are by-passed around the resistors 35 and 36 through the condensers 40 and 4|.

From Figs. 2 and 3 it will now be understood that when the tenth harmonic of the tube oscillator ii is not of the same frequency as the output of the crystal'oscillator w, the unidirectional potential of the points 55 and as m the output circuits of the tubes I2 and l3,respectively, varies in opposite phase at the difference frequency betweenthat of the source l0 and that of the tenth It will also be clear quency s zero the voltages E12 and E13 will be sinusoidal and I35 and I will be constant and (ill It will be understood that the difference frequency determined by the fixed frequency of the source l0 and the selected harmonic of thesource' ll may vary over a wide range. It is for this reason that both an audible and visual signal reproducer is provided in the output of the signal amplifier 44. For example, if the difference frequency producing the signal indication is relatively high, for example, of the order of a hundred cycles per second, it is very difllcult to discern uponthe visual indicator 46 but is easily detectable upon the audible indicator 45. 'Thus by using the headphone 45 an operator may ad-'-- just the frequency of the'vacuum tube oscillator ll, so that the signal frequency is gradually? reduced to the point whereit is no longer audible. J g: At this point the signal frequency willbe sufil ciently low, so that it will'be discernible upon the visual tuning indicator 46, and by now using the visual tuning indicator the operator may con tinue to reduce the signal frequency to zero. In this manner the frequency of the vacuum tube oscillator l| may be readily adjusted to a predetermined value by means of the fixed frequency of the source l0, and the intermediate output frequency at the transformer l4 maintained at a desired value.

While I have described only one preferred embodiment of my invention by way of illustration, many further modifications will occur to those skilled in the art, and I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of maintaining constant the frequency of electric oscillations comprising beating electric oscillations of a predetermined normal fundamental frequency, subject to variation with electric oscillations at a fixed frequency which is. an integral multiple of said fundamental mental frequency to maintain said signal indication fixed.

2. In combination, a pair of electric discharge devices having current control'elements, balanced output circuits for said discharge devices connected in opposing relation, each of said output circuits including a resistor, said resistors being of substantially the same resistance and each having one end connected to a common portion of said output circuits, a first source of electric oscillations having points of like potential connected to a current control element of each of said electric discharge devices, a second source of electric oscillations having points of opposite potential connected to a current control element of each of said electric discharge devices, the frequency of one of said oscillations be'ing fixed and the frequency of a'predetermined harmonic of the other of said oscillations being normally substantially equal to said fixed frequency, energy translating means in said output circuits responsive to the difference of said fixed frequency and the fundamental of said harmonic frequency, and signal reproducing means connected to electrically spaced points of normally equal potential uponsaid resistors for response in accordance with a difference of frequency between said fixed frequency and 'said harmonic frequency, whereby by maintaining a null signal the output of said energy translating means may be maintained at a predetermined frequency.

3. In combination, a pair of electric discharge devices having current control electrodes, balanced output circuits for said discharge devices connected in opposing relation, each of said output circuits including a resistor, said resistors being of substantially the same resistance and each having one end connected to a common portion of said output circuit, a source of electric oscillations of substantially constant frequency having points of like potential connected to a current control electrode of each of said electric discharge devices, a second source of electric oscillations of variable frequency having points of opposite potential connected to a current control electrode of each of said electric discharge devices, said fixed frequency being an integral multiple of the normal fundamental of said variable frequency, energy translating means in said output circuits responsive to-the difference of said fixed frequency and said fundamental frequency, and signal reproducin means connected across at least a portion of each of said resistors to points of normally equal potential for response in accordance with a difference of frequency between said fixed frequency and a predetermined harmonic of said variable'frequ'ency, whereby by maintaining a fixed signal the output of said energy translating means may be maintained at a predetermined frequency.

4. In combination, a pair of electric discharge opposite potential connected to a current control electrode of ,each of said electric discharge devices, the frequency of one of said oscillations being fixed and the frequency of a predetermined harmonic of the-other of said oscillations being normally substantially equal to said fixed frequency, energy translating means in said output circuits responsive to the difference of said fixed frequency and the fundamental of said harmonic frequency, a signal amplifier connected across at least a portion of each of said resistors to points of normallyequal potential. and visual and audible signal reproducing means connected to said signal amplifier for simultaneous response in accordance with a difference of frequency between said fixed frequency and said harmonic frequency, whereby said audible signal means is effective upon a large difference frequency and said visual signal means is effective upon a relatively small frequency difference so that said frequency difference may be adjusted substantially to zero'to maintain the output of said energy translating means at a predetermined frequency.

HOWARD M. CROSBY. 

